Over the past few decades computer aided graphic design software and desk top publishing software have become ubiquitous. Such software allows rapid and efficient production of digital image files that can be used to print books, magazines, pamphlets and other types of documents. Frequently such digital image files are printed using digital printing solutions such as electrophotographic printers, laser printers or ink jet printers.
Such digital printing solutions typically use a print head that is capable of printing lines of image picture elements (pixels) across a printing width. These print heads can form pixels having different densities. During printing, image data in an electronic image file is converted into a sequence of lines of printing instructions. The printing instructions include data from which the print head can determine a density to be printed at each of the pixels. The lines are printed sequentially to form a printed image that has an appearance that represents the image data in the electronic image file.
It will be appreciated from this that proper operation of such digital printers requires that the print head responds to printing instructions at the different pixel location in a generally uniform manner. That is, to achieve uniform density output from a digital printer, that the density printed at any individual engine pixel location cannot significantly vary from the density printed at any other individual engine pixel location.
Density non-uniformities in a print can interrupt the continuity of image content in a print and create unacceptable print artifacts. In particular, even subtle non-uniformities that rise in high quality photographic type images and graphics art content can become readily apparent because they interrupt subtle natural variations of photographs and can disrupt the flat fields having the same density that are often found in graphic images and in text.
Factors that contribute to printer non-uniformity vary, depending on the specific printing technology. With a thermal print head, for example, where resistive print elements are linearly aligned along a writing surface, slight mechanical irregularities or additive mechanical tolerance variability can cause some elements to be more effective in transferring heat than others. With a print head that scans optically, such as a laser print head, optical aberrations or fringe effects can mean that light power is less effectively distributed at the extreme edges of the scan pattern than it is in the center of a scan line. In a printing system that uses an array of light-emitting elements, individual elements in the array may vary in the intensity of light emitted. These variations can be induced for example by thermal, mechanical or electrical variations in manufacturing, assembly, alignment, or in use.
These pixel-to-pixel variations can take various forms. In some instances these variations arise as high frequency variations that arise for example where an individual pixel has a density response that is markedly different from the density response of an adjacent pixel. Such variations typically cause image artifacts that form narrow streaks long the process direction of the print known as streaks. In other instances the pixel-to-pixel variations arise as mid-frequency variations where groups of adjacent pixels have a density response that is different from adjacent groups of pixels to form a pattern of areas having of different densities along the process direction. These mid-frequency variations provide areas that are known are known as bands and typically include groups of pixels that have a density response that is meaningfully different from adjacent groups of pixels.
Streaks and bands are objectionable print artifacts. There have been many efforts to provide systems that measure deviations in the density response at individual pixels or groups of adjacent pixels and that correct the operation of a printer to prevent these conditions. For example, there are a wide variety of automatic feedback and adjustment systems that use one form of color or density sampling or another to automatically calibrate the density response of individual picture elements in a print head so that determine adjustments to be made to the operation of a printing system to attempt to limit pixel to pixel image density variations. For example, U.S. Pat. No. 5,546,165 (Rushing et al.) which discloses non-uniformity correction applied in an electrostatic copier, using LED technology in transfer element. In the '165 patent, feedback measurements from a scanned, flat field continuous tone test print are obtained in order to calculate adjustments to individual LED drive currents or on-times.
Similarly, U.S. Pat. No. 5,684,568 (Ishikawa et al.) discloses non-uniformity correction applied in a printer used for developing photosensitive media. Light intensity from an exposure source employing an array of lead lanthanum zirconate titanate (PLZT) light valves controls image density at each pixel. This output light is measured to identify individual light valve elements that require adjustment for non-uniformity. The approach disclosed in the '568 patent corrects behavior of drive electronics for individual light valve elements, either controlling exposure time or light power level. To obtain and adjust non-uniformity data, this approach uses a basic sensor based feedback path.
U.S. Pat. No. 5,997,123 (Takekoshi et al.) discloses non-uniformity correction applied in an inkjet printer, where a transfer element comprises an array of nozzles. Control electronics are adjusted to modify dot diameter by controlling the applied nozzle energy or by modulating the number of dots produced. The approach disclosed in the Takekoshi et al. patent modifies the behavior of drive electronics assembly for individual inkjet nozzles in the printhead array. To obtain and adjust non-uniformity data, this approach uses the basic scanning device based feedback path. U.S. Pat. No. 6,034,710 (Kawabe et al.) discloses non-uniformity correction applied in a photofinishing printing apparatus that employs Vacuum Fluorescent Print Head (VFPH) technology for printheads 16. Again referring to FIG. 1, the approach disclosed in the Kawabe et al. patent modifies the behavior of drive electronics assembly 26 by adjusting the exposure time of individual elements in the VFPH array. To obtain and adjust non-uniformity data, this approach uses a basic sensor based feedback path. U.S. Pat. No. 5,946,006 (Tajika et al.) discloses non-uniformity correction applied in an inkjet printer, where transfer element 36 comprises an array of nozzles. Referring to FIG. 1, correction data goes directly to a printhead. To obtain and adjust non-uniformity data, this approach uses the basic scanning device based feedback path denoted.
U.S. Pat. No. 5,790,240 (Ishikawa et al.) discloses non-uniformity correction applied in a printer using PLZT (or LED or LCD) printing elements as transfer element 36. Referring to FIG. 1, a correction voltage is applied directly to drive an electronics assembly in order to adjust the output amplitude of an individual PLZT array element. Alternately, duration of the drive signal to an individual PLZT array element is adjusted at drive the electronics assembly. To obtain and adjust non-uniformity data, this approach uses a scanning device based feedback path.
U.S. Pat. No. 4,827,279 (Lubinsky et al.) discloses non-uniformity correction applied in a printer where a print head uses an array of resistive thermal elements to form a corresponding array of pixels. Density measurements are obtained for each individual thermal element and are used to determine correction factors. In the '279 patent a number of applied pulses or pulse duration at drive electronics are used in order to achieve uniformity. To obtain and adjust non-uniformity data, this approach uses a basic scanning device-based feedback path. With each of the conventional solutions noted above, non-uniformity correction is applied by making adjustments to drive electronics.
It will be appreciated from this prior art that it is well known to use feedback strategies to measure and modify the density response of individual pixels or groups of pixels pixel location to seek uniformity by way of adjusting each engine pixel response according to a difference from an aim.
Such approaches are particularly well suited to address high frequency and mid-frequency variations. However, these are not particularly well suited to addressing subtle pixel to pixel variations that occur at low frequencies such as pixel to pixel variations that arise as a product of variations that exist across the cross-track direction. Such low frequency variations can create subtle variations in pixel-to-pixel responses that can accumulate in the cross-track direction so as to give rise to meaningful variations in the density in a printed image. For example, the density of individual pixels near one edge of a cross-track direction can exhibit a noticeably different density response when compared to the density response of individual pixels near an opposite edge in the cross-track direction. These density variations are particularly noticeable in the appearance of a flat density field such as a line or other object that extends between the edges.
However, if the above described high frequency and mid-frequency compensation systems are left to address low frequency problems there is the potential that the low frequency variations can cause suboptimal compensation at any or all of these frequencies of variation. This of course can lead to unsatisfactory density responses. Alternatively, where there is no automatic compensation for low frequency problems the operator of the printer is required to visually identify such variations and make appropriate adjustments manually. This requires a great deal of skill.
It often falls to the operator of a digital printer to make manual adjustments that cause a printer to generate a print that, in the opinion of the operator, has an appearance that most accurately represents the appearance of the electronic image. For example, the printing of a photograph of a black cat on a snowy field is often problematical, with the imaging algorithm employed by the camera making the snow appear to be gray rather than white. Corrections to the density can be made by adjusting the digital data. However, it is time consuming to make adjustments to the digital data that is used to generate a print, thus it is difficult to adjust the image data to the characteristics of machine operation. Further, any adjustments that are made to the image data typically require that the image data be reprocessed into printing data in a time consuming raster imaging process.
Alternatively, many of the tools currently available to the operator of a printer to make at press density adjustments are frequently not precise enough to solve density problems that can impact a plurality of adjacent cells. For example, general density and contrast adjustments can be made that can help to minimize the extent to which density variations in an image are apparent. However, to use such approaches can cause the overall image to have an unintended appearance which in itself can be objectionable.
Printers, even when correctly set initially, can come out of adjustment during a print run. For example, in an electrophotographic print engine, the printing process depletes toner from the developer contained in the development station. Additional toner is inputted into the development station from a replenishment reservoir generally located at one end of the development station and the inputted toner is transported across the development station using known means such as paddles or feed augers. The localized depletion and replenishment of toner can result in density variations across the print while printing. Such variations are particularly objectionable as the customer can directly compare one print with another.
What is needed therefore is a new process control approach that enables a printer to effectively compensate for high frequency, mid-frequency and low frequency variations in pixel-to-pixel density response.